Common industrial methods for producing styrene typically include separation and purification processes, such as distillation, to remove unwanted impurities. Unfortunately, purification processes carried out at elevated temperatures result in an increased rate of undesired polymerization. Heat polymerization is rapid. This polymerization is undesirable during many stages of the manufacturing and processing of styrene monomers, as it results not only in a loss of the desired monomer end-product, but a loss of production efficiency caused by polymer formation and/or agglomeration of polymer on process equipment.
To minimize these losses, distillation is generally carried out under vacuum. In addition, inhibitors and retarders are frequently used. Inhibitors include hydroxylamines and stable free radicals. True inhibitors have a termination rate that is greater than 10 times the polymerization propagation rate. These inhibitors completely eliminate polymer formation for a period of time, known as the induction period. Retarders include dinitrobutylphenol and quinone methides. Retarders are not true inhibitors as their systems have no induction period and the termination to propagation rate is less than 10. Instead, retarders keep polymerization at a low, controlled rate.
Hydroxylamines work well at temperatures between 110° C. to 125° C., and stable free radicals work well at temperatures below 110° C. However, styrene is purified at temperatures that vary from one manufacturing plant to another. Moreover, temperatures will also vary with each distillation column at the same manufacturing plant. Thus, temperature ranges will vary from about 75° C. to 125° C. during the typical styrene distillation process, creating strong need for methods that reduce polymerization across the 75° C. to 125° C. temperature range.